Technical Field
The invention generally relates to devices and methods for battery management in the context of Near Field Communication (NFC) telecommunications systems.
Related Art
The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Telecommunications systems (such as cell phones, smart phones, tablets, etc.) often integrate NFC capabilities. An NFC circuit embedded in a cell phone can be used, for example, as a tag reader in order to read external NFC tags, or to emulate a card device.
Card emulation has many applications. Two well-known use cases relate to micro-payment and e-ticketing.
Micro-payment may consist in purchasing items with a cell phone. For example, one may buy very low cost items (e.g. bread) by tapping the cell phone to an NFC reader (e.g. at a bakery), without even having to type a PIN code. The cell phone may subject the transaction to a check of the transaction amount, which should be below a certain threshold in order for the PIN code not to be necessary.
E-ticketing may consist in using a cell phone rather than a conventional NFC ticket in order to enter public transportation.
Both use cases require a high-level of security. It would not be acceptable to let hackers create money or steal money from third parties' cell phones. To this end, NFC support is often associated with a subscriber identity module within the cell phone that is commonly a SIM card. Interactions between the NFC circuit of a cell phone and a SIM card typically occur over a Single Wire Protocol (SWP) interface.
A problem with initial implementations of NFC support in telecommunications systems lies in the fact that when the system is switched off or whenever the battery level is too low, NFC support is no longer available.
Therefore, with initially implemented technologies, users were not able, for example, to enter public transportation or pay with their cell phone when the cell phone was switched off.
In order to overcome this problem, it has been proposed to power the SIM card through the NFC circuit when the battery is low or the cell phone is switched off.
An NFC reader (such as an NFC interface of a Point of Sales terminal, a.k.a POS, at a bakery), with which the user wishes to interact, may actively generate a radio frequency field that can power a passive target. The passive target may be a SIM card connected to an antenna embedded in a cell phone. Accordingly, the SIM card is available irrespective of whether the cell phone in which it is embedded is itself switched on.
However card emulation support requires a special wiring for the SIM card (as it may be alternately powered by the telecommunications system and by the NFC field).
In platform OFF state (i.e. when the telecommunications system is OFF), ETSI standards require that the VCC contact on the SIM card be maintained pulled down.
A first solution proposed to solve the SIM wiring issue is based on an external switch. As shown on FIG. 1, such solution comprises a module called DUAL P-Channel which acts as a switch and enables card emulation support in both platform ON and OFF modes.
In platform ON mode, the SIM card is directly supplied from the chipset (more specifically from the modem embedded in the chipset).
In platform OFF mode, the VSIM contact of the cell phone (corresponding to the VCC contact of the SIM card, i.e. the contact through which power is supplied to the SIM) is maintained pulled down by the modem. Whenever a card emulation transaction is started, the NFC circuit configures the DUAL P-Channel in such a way that the link between the SIM card and the modem is broken, allowing the NFC circuit to supply power to the SIM card directly and securely.
However this first solution is not optimal. This solution requires an external component (the switch “DUAL P-Channel”) between the chipset and the NFC circuit. This external component avoids that current from the NFC circuit leak back to the chipset through the VCC/VSIM contact when the SIM card is powered by the NFC circuit. However this external component may introduce voltage drop on the VSIM contact, and that is sometimes an issue. Indeed, other contacts of the SIM card (such as the IO contact) are directly connected to the chipset and are not affected by such voltage drop, thereby introducing differences between voltage levels on the VCC contact and other contacts of the SIM card. This may be an issue for SIM cards certifications which require consistent voltage on all contacts of the SIM card.
A second solution proposed to solve the wiring issue is based on a simulated OFF mode. In this solution, whenever a cell phone is supposed to enter a power OFF mode due to user action or due to low battery, it enters a simulated power OFF mode (a.k.a fake power off), in which the cell phone seems to be OFF (the screen may be OFF, the modem may be OFF, etc), however the chipset is still ON with limited capabilities (modem is OFF, connectivity is OFF, etc). Very few features are available, primarily those necessary to maintain the SIM card supplied.
In fake power off, the SIM card is always supplied. In this solution, the SIM card cannot be supplied on demand due to micro-payment latency constraints (a micro-payment transaction typically takes a fraction of a second). Instead, in known fake power off implementations, the SIM card is first powered up. Then it negotiates connection parameters (such as supply voltage level and connection speed in bits/s) with the cell phone based on ATR (Answer To Reset, specified in ISO 7816). It then initializes itself, and finally is placed in “clock stop” stop mode (the CLK contact of the SIM card no longer receives a clock signal from the cell phone). Once in clock stop, the SIM card may ensure that its power consumption is below a certain threshold (various standards define a consumption threshold, but often in clock stop mode only). The SIM card may resume operations instantly, simply by receiving a clock signal from the cell phone. The latency constraints of this second solution (preventing on demand power up of the SIM card) come (inter alia) from the fact that in this solution, the NFC circuit has to request the chipset to start its SIM card controller, and this task is time consuming.
A problem with this second solution is that it allows maintaining card emulation for a limited time only. If the battery is almost empty and is no longer able to power the SIM card, despite being in fake power off, then NFC transactions are no longer possible. Ultimately, the fake power off is replaced by a real power off at the latest when the battery is empty.
This second solution typically provides a few extra hours of autonomy when a battery is low, but anything below 24 hours is typically not satisfactory for end users. In addition, this second solution requires very complex software. When the cell phone is switched off (either manually or due to a low battery event), it shut downs its operating system, and reboots the operating system in fake power off mode. This requires state machines and complex management software at operating system level (with two configurations of the operating system).
Therefore it is proposed to seek to improve the situation.